Abstract 4864: Targeted melanoma therapies as radiosensitizers

Abstract

Abstract Introduction: In BRAFV600 mutant melanoma combination of BRAF and MEK inhibitors is associated with a rapid response in approximately 70% of patients, and an improvement in survival. Radiotherapy is used in the treatment of melanoma in multiple clinical settings including stereotactic radiosurgery for brain metastasis, oligo-metastatic extracranial disease and palliative radiotherapy to symptomatic lesions. Preclinical studies, and a small number of clinical case reports suggest the BRAF inhibitor vemurafenib and the MEK inhibitor trametinib leads to increased radiosensitivity. Aims: To determine if BRAF Inhibitors dabrafenib, vemurafenib and encorafenib, alone or in combination with their corresponding MEK Inhibitors, are radiation-sensitising in BRAF-mutant treatment naive melanoma cells. Methods: Three melanoma cell lines harbouring a BRAF V600E mutation were studied: A375, SkMel28 and MM200. The half-maximal inhibitory concentration (IC50) for each cell line was determined by MTS assays for vemurafenib, dabrafenib, encorafenib, and the MEK inhibitors trametinib, binimetinib and cobimetinib. Clonogenic assays were conducted on each cell line. The optimal dose of BRAF and MEK inhibitors was determined from MTS assay and single drug clonogenic assays. Cells were treated with BRAF or MEK inhibitor alone and in combination. Cells were also treated with increasing doses of radiation (X-RAD 320 irradiator, PXI). The mode of cell death and changes in cell morphology were determined by live cell imaging (Wide Field Live Cell Observer, Ziess) for five days following the above treatments. Results:Using Clonogenic assays, we confirmed that radiation sensitivity varies across different melanoma cell lines, with MM200 being significantly more sensitive to radiation then SKMel28. Clonogenic assays demonstrated that vemurafenib, dabrafenib, encorafenib, trametinib and binimetinib increased radiosensitivity across all three-cell lines. Further, the level of radiosensitsation was greater in combination across all three sets, than with either BRAF or MEK inhibitor alone. Using live cell imaging, we were able to confirm this increase in cell death, and visualise differences in the mode of cell death and morphology with these therapies. Conclusions: Vemurafenib, dabrafenib, encorafenib, trametinib and binimetinib were shown in this study to be radiosensitisers in BRAFV600Emutant melanoma cell lines. Radiosensitisation was greatest with combined BRAF and MEK inhibition. Further studies are being conducted to determine if this sensitisation continues after BRAF resistance develops. Citation Format: Jessica L. Smith, Cecilia Chang, Tim Wang, Helen Rizos, Han Shen, Matteo Carlino, Eric Hau. Targeted melanoma therapies as radiosensitizers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4864.